I. Field of the Invention
The present invention relates to devices that can be used to bore a hole in the center of a broken bolt so that an extractor device can engage the broken bolt and remove it from the threaded aperture in which it lays. More specifically, the present invention relates to a device that can be used to bore a hole in a broken bolt regardless of whether the bolt has broken above or below the open end of the threaded aperture in which the bolt lays.
II. Prior Art Description
When a bolt is over-torqued in a threaded aperture, cross-threaded or driven into a non-compatible threaded aperture, it is common for the drive head of the bolt to break off of the shaft of the bolt. This is especially common when power wrenches are used to drive the bolt since the power wrench can break the bolt before the operator realizes that an over torque condition exists. Tool elements that break in a bore occur in other machining operations such as tapping threads into a bore, reaming a bore, drilling a bore or the like. In such situations hardened tools are used that easily fracture if they are over-torqued or otherwise heavily stressed. Accordingly, it is very common to have a tap, reamer or drill bit break in a bore.
When a bolt, tap, reamer, drill bit or other such tool breaks in a bore, the tool either breaks at a point below the surface containing the bore or at a point above the surface containing the bore. The techniques used in removing a broken element either above or below the surface of the bore often differ greatly and traditionally require different tools.
When a broken element extends well above the surface containing the bore, the broken element can simply be engaged with a wrench or pliers and turned. If the broken element is threaded, a locking nut can be threaded onto the shaft of the element, followed by a drive nut. The locking nut is used to hold the drive nut in place and the drive nut is turned with a wrench, thereby unthreading the broken element.
However, when a broken element is broken below the surface containing the bore or at a point very close to the surface, the methods used to remove the broken element become far more complex. One of the oldest methods of removing a broken element involves drilling a hole down the center of the broken element, driving a spline or reamer into the newly formed hole and turning the spline or reamer to unthread the broken element. Such methods and devices are exemplified by U.S. Pat. No. 1,458,076 to Potts, entitled EXTRACTING DEVICE; U.S. Pat. No. 2,684,526 to Hoover, entitled METHOD FOR BACKING OFF BROKEN STUDS; and U.S. Pat. No. 2,121,197 to Jackman, entitled SCREW EXTRACTOR. From these references, it should be seen that once a hole is drilled down the center of a broken element, there are many devices that can be used to quickly and easily unthread the broken element. The hard part of removing a broken element from a bore is drilling the hole down the center of the broken element.
When a bolt, tap, reamer, drill bit or the like breaks in a bore, the surface at the point of breakage is rarely flat. Furthermore, the metal used in the bolt, tap, reamer or drill bit is typically hardened. As a result, it is very difficult to drill a hole in the broken element without the drill bit walking across the surface of the broken element and drilling into the bore wall. To prevent the drill bit from wandering in such a manner, a starting depression is typically made in the broken element with a pointed punch. This operation requires a punch of the proper size and requires a great deal of time and skill to be performed properly. The process is especially difficult for broken elements with narrow diameters that are broken at points deep within the bore.
U.S. Pat. No. 4,831,902 to McClure, entitled BROKEN BOLT EXTRACTOR, shows a prior art device usable for removing a broken element of a comparatively large diameter. The device includes a threaded member with a plurality of off-axis longitudinal holes formed therethrough. The off-axis holes are used to guide a drill bit as the drill bit drills into the broken element. Rods are then passed through the holes into the broken element, whereby the rods join the broken element to the threaded member. The broken element is then removed by engaging and turning the threaded member. The disadvantage of this prior art system is that it is highly complex and difficult to use. Furthermore, the system can only be used on bolts broken well below the surface bore, and the presence of the drill bits within the threaded member damages the threaded member creating a relatively short useful life for the device.
Recognizing the inherent difficulties in drilling holes in broken elements, other systems have been developed that do not require the drilling of holes. One such method includes welding a drive nut member to the broken element. Such devices and methods are exemplified by U.S. Pat. No. 5,125,144 to Clark, entitled EXTRACTOR FOR BROKEN-OFF TAPS AND THE LIKE AND METHOD OF EXTRACTING THE SAME, and U.S. Pat. No. 4,138,909 to Johnson, entitled BOLT EXTRACTOR. In both of these references, a guide is placed into the bore in which the broken element is located. A welding electrode is then passed through the guide and is used to weld the guide to the broken element. The disadvantage of this system is that it requires specialized welding equipment. Also, the bore may be damaged by the heat of the weld and there remains the possibility that the broken element may become permanently welded into place within the bore.
Accordingly, although the process of drilling a hole in a broken element has its disadvantages, it is still the preferred method used in the art. To simplify the methods of drilling a hole in a broken element and engaging the hole with a reamer, drill bits and reamers have been combined into single extraction devices. Such a device is exemplified by U.S. Pat. No. 5,031,487 to Polonsky, entitled BROKEN BOLT EXTRACTOR. In this device a flange is placed over the bore that contains the broken element. A reamer engages the flange as a bit drills a hole in the broken element. The flange is then removed and the reamer engages the hole made by the bit. The disadvantages of this device is that it is very expensive to manufacture and both the reamer and bit must be sized to match the broken element. Accordingly, such a device can only be used on broken elements that are broken within the bore at a point below the surface containing the bore.
A need therefore exists in the art for a device that can be quickly and easily used to create a hole in a broken element regardless of whether that broken element extends out of a bore or not.
There also exists a need in the art for a broken element extractor that is inexpensive and adaptable to a wide range of broken element diameters.